From the perspective of global environmental conservation, fuel cells that have high electric power generation efficiency and do not emit carbon dioxide are being developed. Fuel cells generate electricity through a reaction of hydrogen with oxygen. A fuel cell basically has a sandwich structure and includes an electrolyte membrane (ion-exchange membrane), two electrodes (a fuel electrode and an air electrode), diffusion layers for hydrogen and oxygen (air), and two separators. Various fuel cells have been developed, in terms of types of electrolyte being used, such as phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, alkaline fuel cells, and solid polymer fuel cells.
Among these fuel cells, as compared with molten carbonate fuel cells, phosphoric acid fuel cells and the like, solid polymer fuel cells advantageously (1) have a significantly low operating temperature of approximately 80° C., (2) can have a light and small battery main body, and (3) have a short transient time, high fuel efficiency, and a high output density. Thus, solid polymer fuel cells are one type of fuel cells receiving the greatest attention today as on-board power sources for electric vehicles and compact distributed power sources for home use (stationary type compact electric generators) and for portable use.
Solid polymer fuel cells, on their working principles, generate electricity from hydrogen and oxygen through a polymer membrane. Solid polymer fuel cells have a structure as illustrated in FIG. 1. The solid polymer fuel cell illustrated in FIG. 1 includes a membrane-electrode assembly (MEA having a thickness of several tens to several hundreds of micrometers) 1 disposed between gas diffusion layers 2 and 3 each formed of a carbon cloth or the like, which are disposed between separators 4 and 5. The membrane-electrode assembly (MEA) 1 is composed of a polymer membrane and an electrode material, such as carbon black carrying a platinum catalyst, both being integrated on the front and back sides of the polymer membrane. This is a unit cell i.e. a single cell and generates electro motive force between the separators 4 and 5. Here, the gas diffusion layers are often integrated with the MEA. Several tens to several hundreds of such single cells are connected in series to form a fuel cell stack in practical use.
Each of the separators serves as a partition between the single cells and also needs to have functions of (1) a conductor that carries generated electrons, (2) a channel for oxygen (air) or hydrogen (an air channel 6 or a hydrogen channel 7 in FIG. 1), and (3) a channel for water and exhaust gas (the air channel 6 or the hydrogen channel 7 in FIG. 1).
In order to develop solid polymer fuel cells for practical use, separators to be used should have high durability and electroconductivity. Some practically used solid polymer fuel cells up to now include separators formed of a carbonaceous material, such as graphite. However, such carbonaceous separators have disadvantage that they are liable to break on impact, are difficult to reduce in size, and require high processing costs for the formation of channels. In particular, high costs are the greatest obstacle to the spread of fuel cells. Thus, attempts have been made to use metallic materials, such as titanium alloys, particularly stainless steel, instead of carbonaceous materials.
Patent Literature 1 discloses a technique for using a metal that can easily form a passivation film for a separator. However, the formation of a passivation film results in a high contact resistance and low electric power generation efficiency. Thus, it has been pointed out as problems to be solved that such a metallic material has a higher contact resistance than carbonaceous materials and low corrosion resistance.
In order to solve the problems, Patent Literature 2 discloses a technique for coating a surface of a metallic separator, for example, formed of SUS 304, with gold to reduce contact resistance and increase output. However, it is difficult to prevent the formation of a pinhole in a thin gold coating. A thick gold coating requires an increased cost.
Patent Literature 3 discloses a method for dispersing a carbon powder on a ferritic stainless steel substrate to manufacture a separator having improved electroconductivity. However, a surface treatment of a separator using a carbon powder is also expensive. It has also been pointed out as a problem that a possible flaw made in a surface-treated separator during assembling significantly deteriorates the corrosion resistance of the separator.
Under such situations, the same applicant filed Patent Literature 4, which discloses a technique for directly using a stainless steel as it is and controlling its surface profile to achieve a low contact resistance and high corrosion resistance. A stainless steel sheet described in Patent Literature 4 has an average peak interval of 0.3 μm or less in its surface roughness profile and can have a contact resistance of 20 mΩ·cm2 or less. This technique has made it possible to provide a fuel cell separator material made of a stainless steel. In fuel cell design, however, there is a demand for further improvement in contact resistance characteristics, and it is desirable to consistently have a contact resistance of 10 mΩ·cm2 or less.
In fuel cells, the contact resistance of a positive electrode (air electrode) subjected to a high electric potential tends to increase due to surface degradation. Thus, it is necessary for a separator to maintain a contact resistance of 10 mΩ·cm2 or less for a long time in the operating environment.
A higher area percentage of a portion having a predetermined surface roughness on stainless steel is advantageous to the characteristics described above. However, the manufacture of a stainless steel sheet having a high area percentage of a portion having a predetermined surface roughness requires strict manufacturing condition control and quality control, resulting in an expensive stainless steel sheet. Thus, it is industrially preferable to achieve the desired performance when the area percentage of a portion having a predetermined surface roughness is less than 100% but more than a certain percentage.
Patent Literature 5 discloses a Mo-containing stainless steel having a low surface contact resistance for fuel cell separators. The area percentage of a region having a fine textured structure (micropit) on the steel surface is 50% or more. However, a study by the present inventors shows that such a textured structure mainly composed of pits does not have a low contact resistance for a long time.
In general, fuel cell separators are formed by press forming of a sheet material. It is desirable that the contact resistance not be increased significantly by sliding in contact with a die in press working. In Patent Literature 2 or 3 in which a film is formed on a surface, the film is partly detached by processing, and the detached portion must be subjected to a batch treatment after press working. This unfavorably increases the number of processes, reduces production efficiency, and increases the cost of the separator.